This invention relates to a brushless DC motor used for an axial fan or the like and a bearing holder of a plastic resin material suitable for use for such a brushless DC motor.
Various kinds of brushless DC motors and fan motors have been conventionally used for a personal computer, a business equipment, a copying machine and the like. Such equipments have been extensively decreased in selling price, so that it is highly demanded to decrease a price of a brushless DC motor. Thus, it has been recently carried out that parts or components of the brushless DC motor such as a bearing holder, a housing and the like are formed of a plastic resin material by injection molding.
For example, U.S. Pat. Nos. 4,806,081 and 5,028,216 each disclose techniques of integrally forming a bearing holder and a housing of a plastic resin material by injection molding. U.S. Pat. No. 4,682,065 discloses techniques of separately forming a bearing holder and a housing of a plastic resin material by injection molding and then integrally joining both to each other. The conventional bearing holder thus formed is mounted therein with two bearings in a manner to be coaxial.
The prior art described above in which the bearing holder is made of a plastic resin material permits a price of a brushless DC motor to be reduced. However, the bearing holder made by the prior art has disadvantages.
One of the advantages is that it is substantially difficult to mount the bearings in both end of the bearing holder while keeping axes of the bearings aligned with each other. More particularly, the bearing holder fails to accomplish alignment of the two bearings in a diametric direction thereof with high accuracy. This brings about an increase in vibration of the bearings during rotation of a rotor, leading to deterioration of durability of the bearings.
Another disadvantage is that an increase in stress of bearings due to a variation in dimensions of the bearing holder with time and due to a variation in temperature causes durability of the bearings to be deteriorated. U.S. Pat. No. 4,682,065 discloses a bearing holder including a structure for reducing such stress, wherein a portion of the bearing holder contacted with bearings is decreased in thickness as compared with a portion thereof uncontacted therewith, to thereby reduce the stress. Nevertheless, the bearing holder disclosed fails to significantly reduce the stress.